Hi aniketgupta,
We understand you're having trouble downloading an update on an iPhone. We're happy to help.
Based on the image you attached with your post, it looks like the Download and Install option is grayed out because the device is not connected to a Wi-Fi network. The device is just connected to the cellular data network as indicated by the "4G" icon showing next to the carrier name instead of the Wi-Fi icon. In order to download an update, the iPhone would need to be connected to, and remain connected to a Wi-Fi network until the download and installation completes.
If you're having issues with connecting the iPhone to a Wi-Fi network, you can try these troubleshooting steps: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
If you're able to get connected to a Wi-Fi network, but the Download and Install button remains grayed out, you can try these steps: If your iPhone, iPad or iPod touch won’t update
We hope this helps. Cheers!